In the steel industry, which is characterized by huge facilities and a large energy consumption, a combination of steps in the manufacturing process, socalled "on-line processing", has been developed in order to simplify the process and reduce the energy consumption. In manufacturing of the steel plate (steel strip and thick plate), a thermo-mechanical treatment in the on-line process has been adopted widely. The conventional heat treatment (such as quenching, tempering, etc.) with using apparatus, which are placed separately from the plate rolling line, is considerably decreased.
On the contrary, in the field of the seamless steel pipe manufacturing, most of the pipes are still treated in a so-called "off-line process", wherein apparatus such as a heating furnace, a cooling equipment and a tempering furnace are installed on the separate line, from the pipe manufacturing line, because the requirement for reliability and quality of the pipe product is very severe. Naturally, it is difficult to reduce the energy consumption in such manufacturing process.
A considerably large space is necessary for the off-line process carried out in the conventional factory layout, since differences of processing speed between each independent step of the process require spaces such as a billetyard to keep the billet before piercing, a space for temporal storage of the pipe before heat treatment. Furthermore, the manufacturing cost increases inevitably because of the conveyance, since the materials should be conveyed from a step to another step, transportation means such as a conveyor, a crane, a truck and operation of loading and unloading, are required.
Recently, in the field of the seamless steel pipe manufacturing, there is a tendency to introduce a so-called "direct quenching process", wherein pipes are quenched immediately after hot working with utilization of the heat retained during the hot-working. By use of this process, it is able to reduce the manufacturing cost remarkably, because a heating furnace for quenching is unnecessary.
For example, as disclosed in Publication of Japanese Patent Application (referred to as PJPA here after) Nos. 56-166324, 58-120720, 58-224116, 56020423, 60-033312, 60-075523 seamless steel pipes, including the direct quenching process in which the steel pipes are forcibly cooled immediately after finish rolling are proposed, and some of these methods have been put into practical use. Unfortunately, products obtained through the direct quenching process have a disadvantage, that the toughness and corrosion resistance of those are inferior to those of the conventional products, which are quenched and tempered in the off-line process, because the products treated by the direct quenching usually have a coarser grain size in microstructure than that of the conventional products treated by quenching and tempering in the off-line process.
In the field of the steel plate manufacturing, as mentioned above, various methods of the direct (on-line) heat treatment of the plate after hot rolling are proposed. For example, PJPA Nos. 62-139815, 63-223125 and 64-055335 disclose methods of directly quenching and tempering of steel plates, which have been worked in the non-recrystallization state and then recrystallized in order to refine grain structure. Since these methods require heavy reduction rolling at a relatively low temperature area, i.e., the non-recrystallization area, it is difficult to be applied to steel pipe rolling, which is accompanied by more complex plastic deformation than that of the plate rolling. For example, steel pipe rolling at a temperature lower than 1000.degree. C., that is the non-recrystallization temperature, by a mandrel mill, i.e., a continuous elongating mill, can not be carried out usually because working stress exceeds the capacity of the mill. Even if the rolling could be performed, many problems, such as surface defects of the pipe and difficulty of mandrel bar extraction, which do not appear at rolling of the plate, occur.
On applying the on-line heat treatment to the manufacture process of the seamless steel pipe, a method for grain refining, by use of recrystallization after pipe forming, is disclosed in PJPA No.61-238917. However, since hot working conditions are not specified in this method, there is a possibility that unfavorable grain growth is promoted when this method is carried out in a practical mill line.
Grain refining methods by means of combination of cooling and re-heating are disclosed (refer to, for instance, PJPA Nos. 56-3626, 63-11621, 58-91123, and 58-1041209). In these methods, two or more cycles of a normal transformation from austenite to ferrite and a reverse transformation from ferrite to austenite are applied. Another grain refining method, wherein twice re-heating are performed during rolling and after rolling, is proposed in PJPA No. 58-117832. However, when the transformation occurs at the early step of finish rolling as is in the methods of PJPA No. 56-3626 and 63-11621, the grains become coarse because the heating temperature should be selected in a relatively high range in order to perform the finish rolling. If the heating temperature is not sufficiently high, the finish rolling can not be performed. In the said PJPA Nos. 58-911231, 58-104120 and 4-358023, conditions for the finish rolling are not specified. Therefore, the treatment of "normal-reverse transformation" can not applied to obtain a sufficient effect to refine grains of the steel pipe. Fine grains are surely provided by a method shown in PJPA No. 58-117832, wherein twice re-heatings are performed. However, the cost of this method is higher than that of the conventional off-line quenching and tempering process, since the cost of equipment and treatment become higher.
From the viewpoint of the continuation of manufacturing steps, there are several proposals in order to reduce cost and save space by combination of various kinds of equipment. For example, PJPA No.63-157705 discloses a method of manufacturing a seamless steel pipe, in which a billet having a round cross section (refer to as "round billet" here after) is pierced and then elongated without passing through blooming or forging step. Also, "Tetsu-to-Hagane" vol. 71(1985), No.8, pp. 965-971 discloses a manufacturing apparatus in which a mandrel mill, i.e., a continuous elongating mill, and an extracting sizer, i.e., a finish rolling mill, are directly connected.
In the method disclosed in the said PJPA No.63-157705, however, the temperature of the round billet, before being charged in the furnace and rolling conditions in the piercer, which is the skew-roll piercing mill, are not specified. In the apparatus described in the above-mentioned "Tetsu-to-Hagane", the two mills are only connected directly without any metallurgical consideration. Objects of the connection in the said apparatus are only to extract the mandrel bar from the rolled pipe, by use of the sizer, and to keep quenching temperature of the pipe. Under the present circumstances, an efficient manufacturing method for the seamless steel pipe having fine grain structure, including the online process, and apparatus suitable for the manufacturing method, in which equipment for treatment are organically arranged, do not exist and are scarcely studied.
In a typical manufacturing process of the seamless steel pipe by hot working, a round billet is pierced and rolled to a hollow shell by so-called Mannesmann piercer, which is typical one of the skew-roll piercing mills, the hollow shell is elongated into a pipe by an elongating rolling mill (referred to as "elongator" here after) such as a plug mill, a mandrel mill, etc., and then the pipe is finished, i.e., sized to the product pipe having a predetermined outer diameter by a finish rolling mill such as a sizer, a stretch reducer, etc. Steps of this conventional manufacturing process, from casting of an ingot or billet to the final product making, is roughly classified as follows.
1) A step of billet making as raw material for hollow shell, PA1 2) A step of hot working included piercing, elongating, and finish rolling, PA1 3) A step of quenching and tempering, i.e., a step of heat treatment. PA1 1 a step of producing a billet, having a round cross section, by continuous casting, PA1 2 a step -of cooling the billet to a temperature not higher than the Ar.sub.1 transformation point, and then heating and soaking the billet at a temperature suitable for piercing of the billet, PA1 3 a step of piercing the soaked billet into a hollow shell at a strain rate of not higher than 200/s, PA1 4 a step of rolling the hollow shell into a seamless steel pipe at an average strain rate of not lower than 0.01/s, with a reduction ratio of not lower than 40%, and finishing the rolling at a temperature from 800.degree. C. to 1050.degree. C., by a mill train (a set of rolling mills), in which a continues elongating rolling mill and a finish rolling mill are closely arranged. PA1 5 a step of cooling the seamless steel pipe to a temperature not higher than the Ar.sub.3 transformation point at a cooling rate of not lower than 80.degree. C./min, PA1 6 a step of reheating the seamless steel pipe at a temperature in a range from 850.degree. C. to 1000.degree. C., for a time from 10 seconds to 30 minutes, quenching and then tempering. PA1 M.epsilon.: strain induced by the continuous elongating rolling mill, PA1 S.epsilon.: strain induced by the finish rolling mill, and PA1 Mt: a period of time (second), from the time when the top end of a hollow shell enters the first roll of the continuous elongating rolling mill to the time when it leaves the last roll of the finish rolling mill. PA1 (A) a continuous casting machine for producing a billet having a round cross section, PA1 (B) a billet heating furnace for heating and soaking the billet which has been cast, PA1 (C) a skew-roll piercing mill for piercing the soaked billet to form a hollow shell, PA1 (D) a continuous elongating rolling mill for elongating the hollow shell into a pipe, PA1 (E) a finish rolling mill for the finish rolling of the pipe into a seamless steel pipe having a predetermined size, PA1 (F) a complementary heating furnace for heating, keeping or slow cooling of the seamless steel pipe after finish rolled, PA1 (G) a heat treating equipment for quenching and tempering the seamless steel pipe.
Each step of above 1) to 3) is usually independent of others. As mentioned above, there are some proposals to combine the steps 2) and 3) continuously, and to perform the so-called on-line process. The typical one is the said direct quenching process.
However, grains of the steel, which are thermo-mechanically treated in a simple direct quench process, tend to be coarser than those of the steel which is treated in the conventional off-line heat treatment (quenching and tempering). Furthermore, it is difficult in the direct quenching process to mass-produce a seamless steel pipe having homogeneous properties steadily. The reasons is that mechanical properties such as strength of the pipe, which is manufactured in the on-line process, vary largely depending on locations in the circumference direction or in the longitudinal direction, because of fluctuation of temperature between positions of a pipe or pipes of different -manufacture lots.
The inventors have proposed, in Japanese Patent Application No. 6-255088 and PCT/JP95/02 155, a method of manufacturing a seamless steel pipe, which is characterized by specifying conditions of hot working and making grains fine by a recrystallization treatment after pipe forming. This method is epoch-making because, in spite of an on-line process for pipe manufacturing, the quality of the steel pipe manufactured by this method is comparable to or superior to that of the pipe manufactured by the conventional off-line heat treatment. Sometimes, however, the method does not fully satisfy the requirement for the seamless steel pipe having higher strength and further improved toughness.